Optical recording medium and disc cartridge

ABSTRACT

An optical recording medium is provided which has a substrate and a signal recording layer provided on the substrate and in which an information signal is recorded in the form of microscopic pits. A light beam is focused on the signal recording layer and some change of the light beam carried by a return light from the signal recording layer is detected to read the information signal recorded on the signal recording layer. The optical recording medium has a diameter of 65 mm or less and a thickness of 0.4 to 0.7 mm. An information signal recording area extends outwardly from a radial position of 12.5 mm or less from the center of the medium. The ratio in area between the information signal recording area and non-signal recording area is 3.4 or more. The storage capacity of this medium is 2 gigabytes or more. The optical recording medium has formed at the center thereof a center hole around which an annular table-abutment convexing to one side of the medium is formed. A magnetic-clamping hub is installed in the center hole and thus the medium is magnetically clamped to a rotation driving mechanism of an optical recording and/or reproducing apparatus. The compactness and high density of recording of the optical recording medium will contribute greatly to a further compact structure of a recording and/or reproducing apparatus in which the medium is to be used.

RELATED APPLICATION DATA

[0001] The present application claims priority to Japanese PatentApplication No. P11-176029, the disclosure of which is fullyincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to an optical recording mediumincluding a substrate and a signal recording layer provided on thesubstrate and in which an information signal recorded on the signalrecording layer is reproduced optically or by detecting a change of thereflected light focused onto the signal recording layer, and to a disccartridge encasing the optical recording medium.

[0003] As one of the conventional recording media for audio and videoinformation, optical discs, from which recorded information is readusing a light beam or to which information is written using a lightbeam, are widely used. Since such an optical disc is formed from asingle plate-like substrate, it can easily be handled and has a largerstorage capacity than other recording media such as magnetic tapes, etc.Therefore, the optical discs are widely used as media for recordingaudio and video information, computer-processed data, etc.

[0004] Information processing units, such as computers, have beendesigned more and more compact with an increasingly smaller internalspace of installation for a recording and/or reproducing apparatus usingan optical recording medium such as an optical disc or the like. Thatis, there is a demand for a smaller design of such a recordingand/reproducing apparatus.

[0005] There are various types of portable or carry-on recording and/orreproducing apparatuses for recording and/or reproduction of audio andvideo information. For a higher portability, it is also demanded thatthe apparatuses be of a more compact design.

[0006] For such a smaller recording and/or reproducing apparatus, arecording medium used in the apparatus should be correspondingly smallerwhile having a larger storage capacity.

SUMMARY OF THE INVENTION

[0007] It is therefore an object of the present invention to provide anovel optical recording medium designed smaller while having a largerstorage capacity, and a disc cartridge in which the optical recordingmedium is encased.

[0008] It is another object of the present invention to provide anoptical recording medium having a larger storage capacity whilecontributing to a more compact design of a recording and/or reproducingapparatus in which the optical recording medium is used, and a disccartridge encasing the optical recording medium.

[0009] It is a further object of the present invention to provide anoptical recording medium designed smaller and positively mountable on arotation driving mechanism to be rotatable along with the rotationdriving mechanism, and a disc cartridge encasing the optical recordingmedium.

[0010] In an embodiment, an optical recording medium according to thepresent invention includes a substrate and a signal recording layerprovided on the substrate, an information signal being recorded on thesignal recording layer. The optical recording medium has a diameter of65 mm or less and a thickness of 0.4 to 0.7 mm. An information signalrecording area extends outwardly from a radial position of 12.5 mm orless from the center of the medium. A ratio between the informationsignal recording area and a non-recording area is 3.4 or more. A storagecapacity is at least 2 GB (gigabytes) or more.

[0011] The above optical recording medium has formed at the centerthereof a center hole around which there is formed an annulartable-abutment being convex to one side of the medium and in which a hubor clamping plate is fixed.

[0012] In an embodiment, an optical recording medium according to thepresent invention includes a substrate and a signal recording layerprovided on the substrate, wherein an information signal recorded on thesignal recording layer is reproduced by detecting a change of thereflected light focused onto the signal recording layer. The opticalrecording medium has a diameter of over 38 mm and under 52 mm and athickness of 0.4 to 0.7 mm. An information signal recording area extendsoutwardly from a radial position of 12.5 mm or less from the center ofthe medium. A storage capacity is at least 1 GB or more.

[0013] The above optical recording medium has formed at the centerthereof a center hole around which there is formed an annulartable-abutment being convex to one side of the medium and in which a hubor clamping plate is fixed.

[0014] In an embodiment, an optical recording medium according to thepresent invention has an annular table-abutment for placement on a disctable of a rotation driving mechanism, and a hub which is to beattracted by a magnet disposed on the disc table, whereby the medium canbe rotated along with the disc table.

[0015] In an embodiment, an optical recording medium according to thepresent invention is a read-only optical recording medium. It includes asubstrate, formed to a thickness of 0.4 to 0.6 mm by injecting resinwithin a mold prepared by stamping a stamper having formed thereon a pitpattern in which the recording track pitch and smallest-pit length areabout 1.4 times larger than those in a first optical recording mediumwith a storage capacity of at least 2 GB including a substrate, and asignal recording layer provided on the substrate, and in whichinformation signal recorded on the signal recording layer is reproducedby detecting a change of the reflected light focused onto the signalrecording layer; and a light-transparent sheet which is 0.05 to 0.1 mmin thickness and having on one side thereof a pit pattern, formed bythermally stamping a pit-pattern stamper, where a signal recording layeris formed. In the optical recording medium, the substrate and sheet arejoined to each other with their respective pit-pattern sides placed in aface-to-face relation through a light-transparent layer of 20 to 35 μm.The first optical recording medium having a diameter of 65 mm or lessand a thickness of 0.4 to 0.7 mm, an information signal recording areadefined thereon extending outwardly from a radial position of 12.5 mm orless from the center of the medium, a ratio in diametrical length of 3.4or more between the information signal recording area and anon-recording area, and a storage capacity of at least 2 GB. It has adiameter of 65 mm or less and a thickness of 0.4 to 0.7 mm. Aninformation signal-recorded area defined thereon extends outwardly froma radial position of 12.5 mm or less from the center of the medium. Astorage capacity is a half or less of that of the first optical discrecording medium.

[0016] In an embodiment, the optical recording medium is a read-onlytype. It has a diameter of over 38 mm and under 52 mm and a thickness of0.4 to 0.7 mm. An information signal recording area extends outwardlyfrom a radial position of 12.5 mm or less from the center of the medium.

[0017] In an embodiment, a disc cartridge according to the presentinvention comprise an optical disc including a substrate; a signalrecording layer provided on the substrate and in which an informationsignal recorded on the signal recording layer is reproduced by detectinga change of the reflected light focused onto the signal recording layer,the optical disc having a diameter of 65 mm or less and a thickness of0.4 to 0.7 mm, an information signal recording area defined thereonextending outwardly from a radial position of 12.5 mm or less from thecenter of the optical disc, a ratio of 3.4 or more between theinformation signal recording area and a non-signal recording area, and astorage capacity of at least 2 GB or more; and a rectangular cartridgein which the optical disc is encased. In the disc cartridge, a writingand/or reading access opening is formed in a side thereof to which anoptical pickup to focus a light beam on at least the optical disc isopposite through which a radially extending portion of the optical discis exposed, and a central opening is also formed through which a centerhole formed at the center of the optical disc is exposed. The writingand/or reading access opening is formed deviated to one lateral sidefrom the middle point of the front side of the cartridge.

[0018] In an embodiment, a disc cartridge according to the presentinvention encases an optical disc including a substrate and a signalrecording layer provided on the substrate, and in which an informationsignal recorded on the signal recording layer is reproduced by detectinga change of the reflected light focused onto the signal recording layer.In an embodiment, a disc cartridge has a diameter of over 38 mm andunder 52 mm and a thickness of 0.4 to 0.7 mm. An information signalrecording area extends outwardly from a radial position of 12.5 mm orless from the center of the medium. Its storage capacity is at least 1GB.

[0019] A disc cartridge according to the present invention encasing theabove-mentioned read-only optical disc has a reading access openingformed only in a lower half.

[0020] These objects and other objects, features and advantages of thepresent intention will become more apparent from the following detaileddescription of the preferred embodiments of the present invention whentaken in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a perspective view of a first optical disc according tothe present invention, showing the upper side thereof.

[0022]FIG. 2 is a perspective view of the optical disc in FIG. 1,showing the rear side thereof.

[0023]FIG. 3 is a sectional view of the optical disc in FIG. 1.

[0024]FIG. 4 is a sectional view of the optical disc in FIG. 1, set onthe rotation driving mechanism of a disc recording and/or reproducingapparatus.

[0025]FIG. 5 is a sectional view of the optical disc in FIG. 1, showingthe signal recording layer.

[0026]FIG. 6 schematically illustrates reading of information signalrecorded on the optical disc in FIG. 1.

[0027]FIG. 7 is a sectional view of a second optical disc according tothe present invention.

[0028]FIG. 8 is a sectional view, enlarged in scale, of a third opticaldisc according to the present invention, this optical disc being of aread-only type.

[0029]FIG. 9 schematically illustrates the process of manufacturing asheet from which the read-only optical disc is produced.

[0030]FIG. 10 is a sectional view of the read-only optical disc, showinghow the substrate and sheet thereof are joined together.

[0031]FIG. 11 is a sectional view of An embodiment of the read-onlyoptical disc according to the present invention.

[0032]FIG. 12 is a perspective view of a first disc cartridge in whichthe first optical disc is encased.

[0033]FIG. 13 is a perspective view of the first disc cartridge, showingthe rear side thereof.

[0034]FIG. 14 is a perspective view of a second disc cartridge encasingthe second optical disc.

[0035]FIG. 15 is a perspective view of the second disc cartridge,showing the rear side thereof.

[0036]FIG. 16 is a plan view of an adapter used to allow the second disccartridge to have same dimensions as the first disc cartridge.

[0037]FIG. 17 is a sectional view of the adapter in FIG. 16.

[0038]FIG. 18 is a plan view of the second disc cartridge set in theadapter.

[0039]FIG. 19 is a perspective view of an optical recording and/orreproducing apparatus in which the optical disc according to the presentinvention is used.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0040] Referring to FIGS. 1, 2 and 3, there is schematically illustrateda first optical disc according to the present invention. The opticaldisc is generally indicated with a reference 1. As shown, the opticaldisc 1 includes a substrate 2 made of glass or a light-transparentsynthetic resin such as polycarbonate resin to be 65 mm or less indiameter and 0.4 to 0.7 mm in thickness, and having a signal recordinglayer formed on one side thereof.

[0041] More specifically, the optical disc 1 consists of the substrate 2made of polycarbonate resin to have a diameter R₁, of 64 mm and athickness D₁ of 0.6 mm as shown in FIG. 3. The substrate 2 has formedtherein at the center thereof a center hole 3 having a diameter R₂ of 8mm. The substrate 2 has formed around the center hole 3 an annulartable-abutment 4 being convex to one side of the substrate 2. Thetable-abutment 4 is to be placed on a disc table of a rotation drivingmechanism in an optical recording and/or reproducing apparatus in whichthe optical disc 1 is set. The table-abutment 4 has an outside diameterR₃ of 14 mm, and is convex approximately D₂ equal to the thickness D₁ ofthe substrate 2 from one side of the latter.

[0042] There is provided at the center of the substrate 2 a magneticclamping hub 5 to close the center hole 3. The hub 5 is formed from athin plate of a magnetic metal such as iron. As shown in FIG. 3, the hub5 consists of a depressed portion 6 formed at the center thereof forfitting into the center hole 3, and a flange portion 7 formed around thedepressed portion 6 for fixing to the substrate 2. The depressed portion6 thus has the form of a bottomed cylinder. The depressed portion 6 hasformed therein at the center thereof a spindle hole 8 in which a spindleof the rotation driving mechanism engages. The spindle hole 8 has adiameter R₄ of 2.0 mm. The table-abutment 4 is concaved as indicatedwith a reference 9 in its convexing direction.

[0043] As shown in FIG. 3, the hub 5 is secured to the substrate 2 withthe depressed portion 6 thereof fitted in the center hole 3 of thesubstrate 2 and the flange portion 7 engaged in the concavity 9 of thesubstrate 2. The hub 5 is fixed at the flange portion 7 thereof in theconcavity 9 with an adhesive. Also as seen from FIG. 3, the hub 5 isfixed to the substrate 2 so that a lower surface 6 a of the depressedportion 6 positioned at the side of the table-abutment 4 will notprotrude from a lower surface 4 a of the table-abutment 4. That is, theheight of the hub 5 from the lower surface 6 a of the depressed portion6 to the flange portion 7 is designed smaller than the thickness of thetable-abutment 4 of the substrate 2.

[0044] As described above, the optical disc 1 includes the substrate 2provided with the table-abutment 4 and hub 5 is set on a rotationdriving mechanism 11 of an optical recording and/or reproducingapparatus as shown in FIG. 4. The rotation driving mechanism 11 consistsof a spindle motor 12, a spindle 13 driven by the motor 12, and a disctable 14 fixed at an end of the spindle 13 rotatably with the spindle13. The disc table 14 has a disc-abutment 15 formed along an outercircumference thereof and a concavity 16 formed inside the disc-abutment15 and in which an annular magnet 17 is fixed. The magnet 17 is securedto be flush with an upper surface 15 a of the disc-abutment 15 or not toprotrude from the disc-abutment surface 15 a. The optical disc 1 is seton the disc table 14 by placing the table-abutment 4 on thedisc-abutment 15 with the spindle 13 fitted into the spindle hole 8 ofthe hub 5. The hub 5 of the optical disc 1 is attracted by the magnet 17and is thus securely held on the disc table 14. Thus the optical disc 1can be rotated along with the disc table 14.

[0045] The spindle 13 is formed to have generally a same diameter as thespindle hole 8 so that the center of rotation of the optical disc 1 cancoincide with the axis of the spindle shaft 13 for correct setting ofthe optical disc 1 on the disc table 14.

[0046] As aforementioned with reference to FIG. 3, the hub 5 is fixed tothe substrate 2 so that a lower surface 6 a of the depressed portion 6positioned at the side of the table-abutment 4 will not protrude fromthe lower surface 4 a of the table-abutment 4. Therefore, when theoptical disc 1 is set on the disc table 14, it will be in the vicinityof, but not in contact with, the magnet 17. Since the hub 5 is placed inthe vicinity of the magnet 17, the optical disc 1 is attracted towardthe disc table 14 by a large magnetic attractive force and thus can berotated along with the disc table 14. When the optical disc 1 is set onthe disc table 14, the hub 5 will not be in contact with the magnet 17,so that no excessive attractive force will act on the hub 5 and hencethe optical disc 1 can easily be set onto or removed from the disc table14.

[0047] Since the hub 5 is secured to the substrate 2, for the depressedportion 6 thereof to be positioned at the side of the table-abutment 4,the spindle 13 can be engaged in the spindle hole 8 of the hub 5 to asufficient amount even if its protrusion from the disc table 14 issmall. For example, if the spindle 13 is provided with a taper centeringend portion 8 a as shown in FIG. 4, it can be formed to such a heightthat it will not protrude from the optical disc 1 and can center thelatter. By forming the spindle 13 to a limited length or height, therotation driving mechanism 11 can be designed to have a limited heightand hence a recording and/or reproducing apparatus using the lowrotation driving mechanism 11 can be designed thinner.

[0048] The magnet used in the conventional magnetically clampingrotation driving mechanism has a strength BHmax of about 30 MG oersteds.In this embodiment, the optical disc 1 includes the substrate 2 made ofpolycarbonate resin to have the diameter R₁ of 64 mm and thickness D₁ of0.6 mm. To keep the optical disc 1 magnetically clamped to the disctable 14 from easily being separated from the disc table 14 even whenthe latter is applied with a shock, a magnetic attractive force of 120 gor so is required. With such an attraction, the optical disc 1 can berotated together with the disc table 14 even at a rotating speed of4,000 rpm or more.

[0049] To obtain such a magnetic attractive force using the magnet usedin the conventional rotation driving mechanism, the hub 5 should have asurface of 8 mm or so in diameter to be attracted by the magnet. Also,to hold the optical disc 1 having the diameter R₁ of 64 mm horizontallyon the disc table 14, the annular table-abutment surface 4 a around thehub 5 should be 2.6 mm or so wide. However, it is difficult for thereason of molding technique to form the table-abutment 4 convex to oneside of the substrate 2 and having a high flatness over its whole lowersurface 4 a to every corners. More specifically, a range of 0.2 mm or soon either end of the table-abutment 4 cannot be counted as thetable-abutment surface 4 a. Taking these factors in consideration, thetable-abutment 4 should be formed to a diameter of at least 14 mm or soin case the hub 5 of 8 mm or so in diameter, which is to be attracted bythe magnet 17, is disposed at the center of the table-abutment 4 andinside the lower or table-abutment surface 4 a of 2.6 mm or so indiameter.

[0050] As having been described above, the optical disc 1 has formed atthe center thereof the table-abutment 4 convex to one side of thesubstrate 2 and in which the hub 5 is to be fixed. Further the opticaldisc 1 has a signal recording layer 21 formed on the other side of thesubstrate 2 as shown in FIG. 5, for example. The signal recording layer21 consists of a recording layer 22, auxiliary recording layer 23,non-magnetic layer 24 and perpendicular magnetic layer 25 stacked inthis order on the substrate 2.

[0051] The recording layer 22 is formed from a film of a material, suchas GdFeCo alloy, having a magnetic axis parallel to the layer surface atroom temperature and which will be magnetized in a directionperpendicular to the layer surface at a temperature approximate to areading temperature attained when heated with a light beam focusedthereon for reading an information signal recorded on the optical disc1. The auxiliary recording layer 23 is provided to improve the readingresolution and thus the reading characteristic and is formed from a filmof a material, such as GdFe alloy, having a Curie temperatureapproximate to the reading temperature attained when heated with thereading light beam, having a magnetic axis parallel to the layer surfacebefore it is heated up to the Curie temperature, and losing the magneticaxis parallel to the layer surface after it is heated up to a highertemperature than the Curie temperature. More particularly, at a lowertemperature than the Curie temperature, the recording layer 22 has aswitched connection with the auxiliary recording layer 23 and thus hasan intra-plane magnetized status in which the direction of magnetizationis parallel to the layer surface. On the other hand, at a temperaturehigher than the Curie temperature, the auxiliary recording layer 23loses the direction of magnetization and thus the recording layer 22 hasa magnetostatic connection with the perpendicular magnetic layer 25 andis perpendicularly magnetized in the magnetized direction of therecording magnetic layer 25.

[0052] The non-magnetic layer 24 is provided to reduce the influence ofthe recording layer 22 on the recording magnetic layer 25 when aninformation signal is written to the recording magnetic layer 25. It isformed from an AIN or SiN film, for example.

[0053] The recording magnetic layer 25 is an amorphous ferromagneticlayer of which the magnetic easy axis is perpendicular to the layersurface. It is formed from a film of an amorphous ferromagnetic materialhaving a large coercive force at room temperature and a Curie pointapproximate to 200° C., such as a TbFeCo alloy or a TbFeCo alloy towhich an element of the fourth group such as Cr is added in a very smallamount.

[0054] The signal recording layer 21 constructed as described above hasa reflective metal layer 26 formed thereon. The reflective metal layer26 is made of a high reflectivity, nonmagnetic metal to reflect 70% ormore of the light beam incident upon the optical disc 1. This materialshould preferably be a good thermal conductor such as Al, Au, Ag or thelike. The reflective metal layer 26 is protected by a protective layer27 of a photo-curing resin or the like formed on the reflective metallayer 26.

[0055] To write an information signal to the signal recording layer 21of the optical disc 1 constructed as above-mentioned, pulse-irradiatedmagnetic modulation is used in which a light beam is pulsed for eachrecording data clock to modulate the recording magnetic field accordingto the recording data. This technology enables a higher density ofrecording than the conventionally used light intensity modulation. Morespecifically, with the pulse-irradiated magnetic modulation, pits havinga same shape as a beam spot focused by an objective lens on the signalrecording layer 21 are written to overlap one on the other on therecording magnetic layer 25, whereby an information signal can berecorded beyond an optical limit depending upon a light spotdistribution (λ/NA) determined from a relation between a wavelength λ ofa light beam generated in the light intensity modulation and a numericalaperture NA of the objective lens by which the light beam is focusedonto the signal recording layer.

[0056] To write an information signal using pulse-irradiated magneticmodulation, a light beam of 380 to 420 nm or 630 to 670 nm in wavelengthλ is used. An objective lens having a numerical aperture NA of 0.5 to0.7 is used to focus the light beam onto the signal recording layer.Recording tracks with a pitch of 0.3 to 0.55 μm are formed to write theinformation signal along them and with a pit length of 0.1 to 0.22μm/bit.

[0057] When the numerical aperture NA of the objective lens is 0.6 andthe working distance between the objective lens and optical disc 1 is1.1 mm, a light beam emitted from a semiconductive laser and formed tobe a parallel light incident upon the objective lens has a diameter of 3mm. In this case, the objective lens upon which the parallel light of 3mm in diameter is incident should have a diameter of 4 mm. Thus, a lensbobbin supporting the objective lens has to be 5 mm or more in diameter.To control the focusing and tracking of the laser beam outgoing from theobjective lens supported by the lens bobbin and which scans the signalrecording area of the optical disc 1, the lens bobbin is installed on abiaxial actuator which drives to move the objective lens in a focusingdirection parallel to the optical axis of the objective lens and in atracking direction perpendicular to the optical axis. Therefore, a spaceof about 1.5 mm for part installation has to be provided outside thelens bobbin.

[0058] The optical disc 1 has the table-attachment 4 of 14 mm or so indiameter provided at the center thereof. The recording and/orreproducing apparatus has disposed therein the disc table 14 havinggenerally the same size as the table-attachment 4 and on which thetable-attachment 4 is to be mounted. When the optical disc 1 constructedas mentioned above is scanned with a light beam emitted from an opticalpickup unit using the objective lens of 4 mm in diameter, the center ofthe light beam can be moved to a radial position of 11 mm near thelead-in area of the optical disc 1. That is, the light beam cannot beirradiated to an inner area having a diameter R₅ of 22 mm. Therefore,the optical disc 1 has an information signal recording area definedoutside the inner area having the diameter R₅ of 22 mm.

[0059] To assure accurate optical read, by a light beam, of aninformation signal recorded as magneto-optical signal in the informationsignal recording area, a birefringence by the substrate 2 has to be 30nm or less for reciprocation of the light beam. Therefore, theinformation signal recording area is defined outside the inner areahaving the diameter R₅ of 22 mm and in which the birefringence of lightbeam indicated upon the substrate 2 is assured to be less than 30 nm forreciprocation.

[0060] As shown in FIG. 3, the optical disc 1 has a signal recordingarea extending outwardly from a radial position of 12.5 mm or less fromthe center of the optical disc 1, namely, from a diametrical position R₆of 25 mm or less, to positively write a maximum amount of an informationsignal to the optical disc 1 and also to positively read a recordedinformation signal from the optical disc 1.

[0061] More specifically, the optical disc 1 has defined thereon anaudio and video information signal recording area of which the insidediameter R₆ is 24.5 mm and the outside diameter R₇ is 61 mm. Further, onthe optical disc 1 are formed, inside the signal recording area, an areain which control signals, etc., which are used for information signalwrite to and read from the optical disc 1, are recorded, and, outsidethe signal recording area an area in which control signals, etc., whichare indicative of the end of the signal recording area, are recorded.

[0062] In the optical disc 1 constructed as in the above, the ratiobetween the information signal recording area and non-information signalrecording areas is 3.4 or more.

[0063] If pulse-irradiated magnetic modulation is adopted with theoptical disc 1 according to the present invention, use of a light beamof 400 nm in wavelength λ permits to recording of record 4 gigabytes ormore of an information signal, while use of a light beam of 660 nm inwavelength λ permits to recording of 2 gigabytes or more of aninformation signal.

[0064] Since it is possible to record 2 gigabytes or more of aninformation signal on the optical disc 1, conventionally proposed signalcompression technology can be used to record many pictures. For example,the JPEG standard can be used to record 200 or more still pictures, andthe audio coding technique included in the MPEG2 standard can be used torecord an MPEG2 moving picture (transfer rate of 8 Mbps) for more than30 min along with sound.

[0065] To read an information signal recorded on the optical disc 1using pulse-irradiated magnetic modulation as described above, a lightbeam L₁ is irradiated to the optical disc 1 through the substrate 2 asshown in FIG. 6. When the light beam L₁ is irradiated to the opticaldisc 1, a portion of a recording layer 22 formed from a GdFeCo alloyfilm on which the light beam L₁ is focused is heated to a highertemperature than predetermined. A portion of the auxiliary recordinglayer 23 formed from the GdFe alloy film on which the light beam L₁ isfocused is heated to a higher temperature than the Curie temperature andloses the direction of magnetization. As a result, the heated portion ofthe recording layer 22 has a magnetostatic connection with the recordingmagnetic layer 25 and is perpendicularly magnetized in the magnetizeddirection of the recording magnetic layer 25 as shown in FIG. 6A. Thesurrounding of the perpendicularly magnetized portion of the recordingmagnetic layer 25 keeps an intra-plane magnetized status in which themagnetic axis is parallel to the layer surface to function as a mask 29in which no Kerr effect will take place while the perpendicularlymagnetized portion functions as an aperture 30 for the light beam L₁incident upon the recording magnetic layer 25, as shown in FIG. 6B.Thus. since only the portion of the recording layer 22 on which thelight beam L₁ is focused is perpendicularly magnetized along themagnetized direction of the recording magnetic layer 25, it is possibleto read, with high resolution, an information signal recorded in theform of microscopic pits 32 on recording tracks 31 formed with a smallpitch on the optical disc 1.

[0066] In this embodiment, the first optical disc 1 has the diameter R₁of 64 mm and the thickness D₁ of 0.6 mm. However, the present inventionis not limited to these dimensions, but the diameter R₁ may be increasedup to 65 mm. The diameter R₁ may be 65 mm or less so long as an intendedstorage capacity can be attained. Also, the thickness D₁ of the opticaldisc 1 may appropriately be selected from a range of 0.4 to 0.7 mmdepending upon the wavelength of a light beam used to write and/or readthe information signal.

[0067] In addition to the first optical disc 1 as described in theforegoing, the present invention provides also another embodiment of anoptical disc.

[0068] Referring now to FIG. 7, there is schematically illustrated asecond optical disc according to the present invention. The optical discis generally indicated with a reference 41. This optical disc 41 can beused in an optical recording and/or reproducing apparatus in which theaforementioned optical disc 1 can also be used. The optical disc 41 hasthe same construction as the optical disc 1 except for its outsidediameter. Therefore, the same or similar elements as or to those of theoptical disc 1 are indicated with the same or similar references andwill not further be described.

[0069] The optical disc 41 is formed to have a diameter of more than 38mm to less than 52 mm and a thickness of 0.4 mm to 0.7 mm. Morespecifically, the optical disc 41 has a diameter R₁₁ of 50 mm as shownin FIG. 7. The optical disc 41 comprises a substrate 42 formed fromglass or a light transparent polycarbonate resin having a signalrecording layer formed on one side thereof.

[0070] The substrate 42 of the optical disc 41 is formed from apolycarbonate resin to have the diameter R₁₁ of 50 mm and a thicknessD₁₁of 0.6 mm as shown in FIG. 7. The substrate 42 has formed at thecenter thereof a center hole 3 having a diameter R₂ of 8 mm as in theoptical disc 1. The substrate 42 has formed around the center hole 3 anannular table-abutment 4 being convex to one side of the substrate 42.There is provided at the center of the substrate 42 a magnetic clampinghub 5 to close the center hole 3 as in the optical disc 1. As shown, thehub 5 consists of a depressed portion 6 formed at the center thereof.The depressed portion 6 has formed therein at the center thereof aspindle hole 8 in which engages a spindle of a rotation drivingmechanism. The spindle hole 8 has a diameter R₄ of 2.0 mm. As in theoptical disc 1, the hub 5 is fitted at the depressed portion 6 thereofin the center hole 3 of the substrate 2, and the table-abutment 4 isconcaved as indicated with a reference 9 in its convexing direction. Thehub 5 has a flange 7 formed around the depressed portion 6 thereof andwhich is to be engaged in the concavity 9 of the substrate 42.

[0071] The substrate 42 has provided on the other side thereof, oppositeto that where the table-abutment 4 is formed, a signal recording layer21, which is similar to that in the optical disc 1.

[0072] To write an audio and video information signal to the signalrecording layer 21 of the optical disc 41, constructed asabove-mentioned, pulse-irradiated magnetic modulation, in which a lightbeam is pulsed for each recording data clock to modulate the recordingmagnetic field according to the recording data, is adopted as with theoptical disc 1.

[0073] As in the optical disc 1, the optical disc 41 has thetable-attachment 4 provided at the center thereof as mentioned above.The recording and/or reproducing apparatus, which is compatible withboth the optical discs 1 and 41, has disposed therein a disc table 14having the generally same size as the table-attachment 4 and on whichthe table-attachment 4 is to be mounted. Thus, the optical pickup unitcompatible with the optical disc 1 will be used with the optical disc42. Therefore, similarly to the optical disc 1, the optical disc 41 willhave formed thereon an information signal recording area extendingoutwardly from an inside diameter R₅ of at least 22 mm. Namely, theoptical disc 41 has a signal recording area extending outwardly from aradial position of 12.5 mm or less from the center of the optical disc41, namely, from a diametrical position R₆ of 25 mm or less, as shown inFIG. 7, to positively write a maximum amount of information signal.

[0074] More specifically, the optical disc 41 has defined thereon anaudio and video information signal recording area of which the insidediameter R₆ is 24.5 mm and the outside diameter R₁₇ is 46.5 mm. Further,on the optical disc 41 are formed, inside the signal recording area, anarea in which control signals, etc., which are used for informationsignal write to and read from the optical disc 1, are recorded; and,outside the signal recording area, an area in which control signals,etc., which are indicative of the end of the signal recording area, arerecorded.

[0075] If pulse-irradiated magnetic modulation is adopted with theoptical disc 41 according to the present invention, use of a light beamof 400 nm in wavelength λ permits recording of 2 gigabytes or more of aninformation signal, while use of a light beam of 660 nm in wavelength λpermits recording of 1 gigabytes or more of an information signal.

[0076] To read an information signal recorded on the optical disc 41using the pulse-irradiated magnetic modulation as described in theabove, the same method as that for reading an information signalrecorded on the optical disc 1 is adopted.

[0077] In the second embodiment, the second optical disc 41 has thediameter R₁₁ of 50 mm and the thickness D₁₁ of 0.6 mm. To assure anintended storage capacity of the optical disc 41, however, the diameterR₁₁, may appropriately be selected from a range of 38 to 52 mm while thethickness D₁₁ may appropriately be selected from a range of 0.4 to 0.7mm depending upon the wavelength of a light beam used for write and/orread of an information signal.

[0078] In the foregoing, the first and second optical discs 1 and 41according to the present invention have been described. Each of theoptical discs 1 and 41 has only a single signal recording layer on eachof the substrates 2 and 42. However, in case the optical discs 1 and 41are read-only optical discs, two or more signal recording layers may beformed on each of the substrates 2 and 42.

[0079] Referring now to FIG. 8, there is schematically displayed a thirdoptical disc, which is a read-only optical disc having two signalrecording layers. The optical disc is generally indicated with areference 111. The optical disc 111 is formed to have the same size asthe first optical disc 1. Its diameter is 65 mm or less and itsthickness ranges from 0.4 to 0.7 mm.

[0080] Similar to the optical disc 1, the optical disc 111 may have adiameter of up to 65 mm. It may have a diameter of 65 mm or lessdepending upon an intended storage capacity. Also the optical disc 111is formed to have a thickness appropriately selected from a range of 0.4to 0.7 mm according to the wavelength of a light beam used to read aninformation signal recorded on the optical disc 111.

[0081] As shown in FIG. 8, the third optical disc 111 comprises asubstrate 112 formed from a light-transparent synthetic resin such aspolycarbonate. The substrate 112 is formed to have a diameter of 65 mmor less and a thickness D₂₁ ranging from 0.4 to 0.7 mm. It has preformedon one side thereof a pit pattern 113 that is a pattern of very smallconvexities and concavities. That is to say, the substrate 112 is formedby injecting a molten polycarbonate into a mold in which there isdisposed a stamper having formed thereon a pattern of convexities andconcavities corresponding to the concavities and convexities,respectively, on the pit pattern 113. As shown in FIG. 8, the substrate112 has a first signal recording layer 114 formed along the pit pattern113 on one side thereof where the pit pattern 113 is formed. The firstsignal recording layer 114 is formed from a translucent silicon filmsuch as Si₃N₄, SiO₂ or the like which allows to pass a predeterminedamount of a light beam irradiated through the substrate 112 whilereflecting a predetermined amount of the light beam. The first signalrecording layer 114 consists of more than one silicone film of Si₃N₄,SiO₂ or the like stacked one on the other to a thickness D₂₂ of 100 to500 nm. The silicon films of Si₃N₄, SiO₂ or the like are produced byvacuum deposition or sputtering.

[0082] In this read-only optical disc 111, the pit pattern 113 formed onthe substrate 112 is formed with a track pitch about 1.4 times largerthan that of the recording tracks formed on the optical discs 1 and 41.Also, the minimum length of the pits included in the pit pattern 113 isabout 1.4 times larger than those in the optical discs 1 and 41. Thatis, in the read-only optical disc 111, the track pitch of the recordingtracks is 0.4 to 0.77 μm and pit length is 0.14 to 0.31 μm/bit.Therefore, the recording tracks can be formed by injection molding of asynthetic resin.

[0083] When formed to sizes for the discs 1 and 41, respectively, thestamper prepared with the above-mentioned track pitch and shortest pitlength will provide a pit pattern being a half of that recorded on theoptical discs 1 and 41. The pit pattern will result in a storagecapacity of the substrate 112 that is also a half of that of the discs 1and 41. Therefore, when the stamper in consideration is used to form theoptical disc 111 in a size corresponding to that of the discs 1 and 41,the amount of information recordable on this optical disc 111 will be ahalf of that recordable on the optical discs 1 and 41.

[0084] In the optical disc 111, the substrate 112 has a center hole 3formed at the center thereof, as in the optical discs 1 and 41. Thesubstrate 112 has formed around the center hole 3 and on a side thereof,opposite to the side on which the signal recording layer 114 is formed,an annular disc-abutment 4 convexing away from the signal recordinglayer 114. Further, a magnetic-clamping hub 5 is provided at the centerof the substrate 112 to close the center hole 3, as in the optical discs1 and 41.

[0085] To the first signal recording layer 114 on the substrate 112 ofthe optical disc 111, there is joined a sheet 116 formed sufficientlythinner than the substrate 112 and having preformed on one side thereofa pit pattern 115 being a pattern of very small concavities andconvexities corresponding to information signals. There is providedbetween the first signal recording layer 114 and sheet 116 alight-transparent layer 117 made of a light-transparentultraviolet-curable synthetic resin.

[0086] The sheet 116 is formed from a light-transparent synthetic resinsuch as polycarbonate to a thickness D₂₃ of 0.05 to 0.1 mm. The pitpattern 115 formed on one side of the sheet 116 is formed by thermallystamping a pattern formed on a stamper and consisting of convexities andconcavities corresponding to the concavities and convexities,respectively, of the pit pattern 115. As shown in FIG. 8, a secondsignal recording layer 118 is formed along the pit pattern 115. Toreflect with a high efficiency towards an optical pickup unit disposedin front of the substrate 112, a light beam focused thereon through thefirst signal recording layer 114 and light-transparent layer 117, thesecond signal recording layer 118 is formed from a film of a highlyreflective metal such as aluminum (Al), gold (Au) or silver (Ag).

[0087] The sheet 116 having the pit pattern 115 and second signalrecording layer 118 formed thereon is produced through a process asshown in FIG. 9. To form the sheet 116, a polycarbonate web 121 is fedto a replicating or stamping post where a stamper 122 and pressingroller 124 are provided. The stamper 122 has formed thereon a pattern123 of concavities and convexities corresponding to the convexities andconcavities, respectively, of the intended pit pattern 115. The pressureroller 124 is provided with a heating means. The polycarbonate web 121is passed between the stamper 122 and pressure roller 124 while beingheated and pressed. Thus, the polycarbonate web 121 has the pit pattern123 stamped thereon, and is further fed towards a take-up roll 121 a.Before the polycarbonate web is taken up on the roll 121 a, it is movedinto a sputtering post where the second signal recording layer 118 isformed on a portion of the web 121 on which the pit pattern 123 has beenstamped. Further, the polycarbonate web 122 is moved to a punching postwhere the web portion, on which the second signal recording layer 118 isformed, is punched to a size for attachment to the substrate 112,thereby forming the sheet 116. The web 122 is wound on a take-up roll121 a.

[0088] The pit pattern 123 formed on the stamper 122 is formed onrecording tracks whose pitch is about 1.4 times larger than that of therecording tracks formed on the optical discs 1 and 41. The minimumlength of the pits forming the pit pattern 115 is also about 1.4 timeslarger than that in the optical discs 1 and 41. When the stamper 122 forsuch tack pitch and minimum pit length is formed to sizes correspondingto those of the optical discs 1 and 41, respectively, it will have a pitpattern being a half of those of the pit patterns formed on the opticaldiscs 1 and 41, and whose storage capacity is also a half of those ofthe optical discs 1 and 41. When the sheet 116 formed by the stamper 122is formed to sizes corresponding to those of the optical discs 1 and 41,respectively, the amount of information recorded on the sheet 116 willbe a half of those of the optical discs 1 and 41.

[0089] The sheet 116 thus formed is joined to the substrate 112 with thesecond and first signal recording layers 118 and 114 placedface-to-face. As mentioned above, an ultraviolet-curable resin is usedas the light-transparent layer 117 between the sheet 116 and substrate112.

[0090] To join the sheet 116 to the substrate 112, they are centeredwith each other and the first and second signal recording layers 114 and118 are placed in a face-to-face relation with a space D₂₄ of 20 to 35μm between them. To this end, a jig 125, for example, is inserted into acenter hole 116 a formed at the center of the sheet 116 and center hole3 in the substrate 112 as shown in FIG. 10 to center the sheet 116 andsubstrate 112 with each other and to provide the space of 20 to 35 μmbetween the first and second signal recording layers 114 and 118. Thenan ultraviolet-curable resin, for example, is injected between thesubstrate 112 and sheet 116 to form the light-transparent layer 117.With the substrate 112 and sheet 116 rotated, the ultraviolet-curableresin injected between the substrate 112 and sheet 116 is spread to auniform thickness. Thereafter, ultraviolet rays are irradiated throughthe substrate 112 to harden the ultraviolet-curable resin to join thesubstrate 112 and sheet 116 to each other.

[0091] Next, the hub 5 is installed to the substrate 112 to cover thecenter hole 3 in the substrate 112 to finish the optical disc 111.

[0092] As mentioned above, the substrate 112 and sheet 116 are joined toeach other with the space of 20 to 35 μm between them and theultraviolet-curable resin, for example, injected into the space to formthe light-transparent layer 117 which will thus have a thickness D₂₄ of20 to 35 μm.

[0093] When a light beam of 380 to 420 nm in wavelength λ, or a lightbeam of 630 to 670 nm in wavelength λ, is focused on the first or secondsignal recording layer 114 or 118 through an objective lens having anumerical aperture (NA) of 0.5 to 0.7, the focal depth will be ±1 μm.Since the light-transparent layer 117 has the thickness D₂₄ of 20 to 35μm, an information signal recorded on the first or second signalrecording layer 114 or 118 can accurately be read by a common light beamL₂ and objective lens 131 independently without mutually affecting eachother and without being influenced by a spherical aberration of theobjective lens 131.

[0094] To read an information signal from the first or second recordinglayer 114 or 118, the position of the objective lens 131 is controlledfor the light beam L₂ to be focused on the first or second signalrecording layer 114 or 118 as shown in FIG. 8. The position of theobjective lens 131 is controlled using, for example, control signalsindicative of the first and/or second signal recording layers 114 and/or118 and recorded in control signal recording areas for the first and/orsecond signal recording layers 114 and/or 118.

[0095] In the aforementioned third optical disc 111 according to thethird embodiment, an information signal can be recorded on each of thefirst and second signal recording layers 114 and 115 in an amount beinga half of that recordable in the optical disc 1. Therefore, the opticaldisc 111 has a total storage capacity equivalent to that of the opticaldisc 1.

[0096] As in the above, the read-only optical disc 111 is constructed sothat an information signal is read from the first and second signalrecording layers 114 and 118 by irradiating light beams through thesubstrate 112 to the respective signal recording layers 114 and 118.However, by forming the second signal recording layer 118 from atranslucent material which will allow to pass a predetermined amount ofa light beam while reflecting a predetermined amount of the light beam,it is also possible to read information signal from the first and secondsignal recording layers 114 and 118 by irradiating a light beam throughthe sheet 116.

[0097] Further, in the read-only optical disc 111, a second sheet 126may be joined to the sheet 116 (namely, the first sheet) with a secondlight-transparent layer 127 made of a light-transparent,ultraviolet-curable material or the like laid between them as shown inFIG. 11. Namely, three signal recording layers may be formed in theoptical disc 111. In this case, the signal recording layer 118 formed onat least the first sheet 116 is formed as a translucent layer which willallow to pass a predetermined amount of a light beam while reflecting apredetermined amount of the light beam.

[0098] The above-mentioned read-only multi-layered optical disc may beformed to the size of the second optical disc according to the presentinvention, that is, the optical disc 41. Also in this case, since eachof the signal recording layers is able to record a half of theinformation amount recordable on the optical disc 41, the two signalrecording layers can record a total amount of information equivalent tothat recordable in the first optical disc according to the presentinvention, namely, the optical disc 1.

[0099] The optical discs 1 and 41 constructed as having been describedin the foregoing record an information signal with an extremely highdensity. Therefore, any little dust or small scratch on the opticaldiscs will make it impossible to accurately write or read an informationsignal.

[0100] Thus, to avoid the above inconvenience, each of the optical discs1 and 41 according to the present invention is encased in a cartridge.As a disc cartridge, it is set as encased in a cartridge in a recordingand/or reproducing apparatus or it is stored as encased in thecartridge.

[0101] Referring now to FIGS. 12 to 19, there are illustrated disccartridges according to the present invention.

[0102] The disc cartridge is generally indicated with a reference 51 asshown in FIGS. 12 and 13. The disc cartridge 51 encases the optical disc1 having been described in the foregoing. It consists of upper and lowerhalves 52 and 53 butted to each other to form a cartridge 54 in whichthe optical disc 1 is encased. The cartridge 54 is formed to have arectangular shape of which a first side has a length W₁ of 52 to 70 mmand a second side has a length W₂ of 52 to 75 mm, perpendicular to thefirst side. Therefore, the optical disc 1 of 65 mm or less in diametercan be encased rotatably in the cartridge 54. The lengths W₁ and W₂ ofthe first and second sides of the cartridge 54 are appropriatelyselected from ranges of 52 to 70 mm and 52 to 75 mm, respectively,according to the size of the optical disc 1 to be encased in thecartridge 54. The cartridge 54 has a thickness D₃ selected from a rangeof 3 to 6 mm according to the thickness of the optical disc 1 to beencased therein.

[0103] That is, the cartridge 54 is formed according to the size of theoptical disc 1 to have a rectangular shape having a necessary minimumlength, width and thickness to encase the optical disc 1.

[0104] In effect, the disc cartridge 51 shown in FIGS. 12 and 13comprises the cartridge 54 with the first side having the length W₁ of68 mm, second side having the length W₂ of 72 mm and thickness D₃ of 5mm since the optical disc 1 to be encased therein has a diameter R₁ of64 mm and thickness D₁ of 0.6 mm.

[0105] The cartridge 54 has formed in the upper half 52 thereof awriting access opening 55 through which a radially extending part of theoptical disc 1 encased therein is exposed. The writing access opening 55is provided to allow an external field-applying magnetic head of arecording and/or reproducing apparatus to approach the optical disc 1for writing of an information signal to the latter. The writing accessopening 55 is formed to a rectangular shape extending from near a centerof the cartridge 54 to near a shorter front side of the cartridge 54.The writing access opening 55 is located with its longitudinal axispassing through the center of the optical disc 1 and a middle point P₁of the front side width of the cartridge 54.

[0106] Also, the lower half 53 of the cartridge 54 has formed therein areading/writing access opening 56 through which a radially extendingpart of the optical disc 1 encased therein is exposed and which allowsan optical pickup unit of a recording and/or reproducing apparatus toapproach the optical disc 1 for writing and reading of an informationsignal to and from the latter, and a central opening 57 through whichthe table-abutment 4 provided at the center of the optical disc 1 isexposed. The central opening 57 is formed at the center of the lowerhalf 53, and the reading/writing access opening 56 is formedcontiguously to the central opening 57 to have a rectangular shapeextending to near the front side of the cartridge 54. As shown in FIG.13, the reading/writing access opening 56 has a middle point P₂ of thewidth thereof deviated from the middle point P₁ of the front side widthof the cartridge 54 towards one of the lateral sides of the cartridge54. Owing to this deviation of the reading/writing access opening 56from the center of the cartridge 54, the main body of the optical pickupunit can be displaced from the center line of the optical disc 1 withthe optical axis of the objective lens positioned on the center line ofthe optical disc 1, so that the recording and/or reproducing apparatuscompatible with the disc cartridge 51 can be designed more compact.

[0107] The cartridge 54 is provided with a shutter member 58 to open andclose the writing access opening 55, reading/writing access opening 56and central opening 57 at the same time. The shutter member 58 is madeby punching and bending a thin metal sheet. It consists of a firstshutter portion 59 to open and close the writing access opening 55,second shutter member 60 to open and close the reading/writing accessopening 56 and central opening 57, and a juncture 61 to connect thebases of the first and second shutter portions 59 and 60. The shuttermember 58 is formed to have a generally C-shaped cross-section. Thejuncture 61 has first and second projecting sliding guides 62 and 63fitted on the front side of the cartridge 54. As shown in FIG. 12, thefirst shutter portion 59 is formed to have a rectangular shape havingsufficient dimensions to close the writing access opening 55. As shownin FIG. 13, the second shutter portion 60 extends from the juncture 61while being deviated laterally in relation to the first shutter portion59 to close both the reading/writing access opening 56 formed off thecenter of the cartridge 54 and the central opening 57 formed at thecenter of the cartridge 54. Also the second shutter portion 60 is bentto close the central opening 57 which is positioned off the longitudinalaxis of the reading/writing access opening 56.

[0108] The shutter member 58 is fitted onto the cartridge 54 with thefirst shutter portion 59 laid over the writing access opening 55 and thesecond shutter portion 60 laid over the reading/writing access opening56 and central opening 57. At this time, the first and second slidingguides 62 and 63 of the shutter member 58 are engaged on the front sideof the cartridge 54 with guide pieces thereof (not shown) engaged insliding guide recesses, respectively, formed in the cartridge 54. Theshutter member 58 thus installed on the cartridge 54 can be preventedfrom slipping off on the cartridge 54, and can be moved along the frontsurface of the cartridge in the directions of arrows A and B in FIGS. 12and 13 to open and close the writing access opening 55, reading/writingaccess opening 56 and central opening 57 simultaneously.

[0109] There is provided inside the cartridge 54 a locking member (notshown) which engages with the shutter member 58 having been moved to aposition where it closes the writing access opening 55, reading/writingaccess opening 56 and central opening 57, to thereby hold the shuttermember 58 in that position. When the shutter member 58 is in the closedposition, the locking member engages with a locking piece provided onthe first sliding guide 62 to keep the shutter member 58 in the closedposition.

[0110] When the disc cartridge 51 is ejected from a recording and/orreproducing apparatus, the shutter member 58 is held by a shutterholding member and moved in relation to the cartridge 54 to a positionwhere it closes the reading/writing access opening 56 and centralopening 57. Therefore, the disc cartridge 51 needs no forcing means suchas a spring to move the shutter member 58 to the closed position.

[0111] The second shutter portion 60 formed long enough to close thereading/writing access opening 56 and central opening 57 has a free endthereof supported by a moving guide piece 60 a fixed on the lowersurface of the cartridge 54 as shown in FIG. 13. Thus the second shutterportion 60 is prevented from floating from the lower surface of thecartridge 54 and can positively close the central opening 57.

[0112] The cartridge 54 has formed on the front side thereof anengagement recess 64 in which a shutter releasing member provided in arecording and/or reproducing apparatus is engaged when the disccartridge 51 is set in the recording and/or reproducing apparatus. Theengagement recess 64 is open at one end thereof at which the disccartridge 51 is first introduced into the recording and/or reproducingapparatus.

[0113] When the disc cartridge 51 thus constructed is introduced into arecording and/or reproducing apparatus first at one lateral side thereofat which the engagement recess 64 is open, namely, in the direction ofarrow E in FIGS. 12 and 13, the shutter releasing member of therecording and/or reproducing apparatus enters the engagement recess 64,and presses to displace the locking member, thereby disengaging thelatter from the locking piece of the shutter member 58. As the disccartridge 51 is further inserted into the recording and/or reproducingapparatus after the locking piece is disengaged from the locking member,the cartridge 54 is moved while the shutter member 58 is restricted bythe shutter releasing member from moving, so that the writing accessopening 55, reading/writing access opening 56 and central opening 57 areopened. Thus the disc cartridge 51 is set in place in the recordingand/or reproducing apparatus.

[0114] As shown in FIG. 13, the cartridge 54 has formed on the lowersurface thereof first and second positioning holes 65 and 66 forreceiving positioning pins provided in the recording and/or reproducingapparatus. The positioning holes 65 and 66 are formed in positions atthe front comers, respectively, of the cartridge 54 at which the holes65 and 66 are not opposite to the optical disc 1 encased in thecartridge 54.

[0115] The first disc cartridge 51 constructed as described in the aboveis formed to have a sufficient size to encase the optical disc 1 havinga small diameter. The reading/writing access opening 56 is formed nearerto one lateral side of the cartridge 54. Therefore, along with thecompact design of the disc cartridge 51, the recording and/orreproducing apparatus in which the disc cartridge 51 is to be used canbe designed correspondingly compact.

[0116] Referring now to FIGS. 14 and 15, there is illustrated a seconddisc cartridge according to the present invention, designed to encasethe second optical disc according to the present invention, namely, theoptical disc 41. The disc cartridge is generally indicated with areference 71.

[0117] Similar to the first disc cartridge, namely, the disc cartridge51 having been described in the foregoing, the disc cartridge 71consists of an upper half 72 and lower half 73 butted to each other toform a cartridge 74 in which the disc cartridge 41 is to be encased. Thecartridge 74 is formed to have a rectangular shape of which a first sidehas a length W₁ of 41 to 57 mm and a second side has a length W₂ of 41to 62 mm, perpendicular to the first side. Therefore, the optical disc41 of more than 38 mm and 52 mm or less in diameter can be encasedrotatably in the cartridge 74. The lengths W₁, and W₂ of the first andsecond sides of the cartridge 74 are appropriately selected from rangesof 41 to 57 mm and 41 to 62 mm, respectively, according to the size ofthe optical disc 1 to be encased in the cartridge 54. The cartridge 54has a thickness D₄ selected from a range of 3 to 6 mm according to thethickness of the optical disc 41 to be encased therein.

[0118] That is, the cartridge 74 is formed according to the size of theoptical disc 41 to have the rectangular shape having minimum necessarylength, width and thickness to encase the optical disc 41.

[0119] In effect, the disc cartridge 71 shown in FIGS. 14 and 15comprises the cartridge 74 with a first side having a length W₃ of 54mm, second side having a length W₄ of 57 mm and thickness D₄ of 5 mm,since the optical disc 41 to be encased therein has a diameter R₁₁ of 50mm and thickness D₁₁ of 0.6 mm.

[0120] As in the disc cartridge 51, the cartridge 74 has formed in theupper half 72 thereof a writing access opening 75 through which aradially extending part of the optical disc 41 encased therein isexposed. Also, the lower half 73 of the cartridge 74 has formed thereina reading/writing access opening 76 through which a radially extendingpart of the optical disc 41 encased therein is exposed and which allowsan optical pickup unit of a recording and/or reproducing apparatus toapproach the optical disc 41 for writing and reading of an informationsignal to and from the latter, and a central opening 77 through whichthe table-abutment 4 provided at the center of the optical disc 41 isexposed. Similarly to the reading/writing access opening in the disccartridge 51, the reading/writing access opening 76 has a middle pointP₄ of the width thereof deviated from the middle point P₃ of the frontside width of the cartridge 74 towards one of the lateral sides of thecartridge 74, as shown in FIG. 15.

[0121] The cartridge 74 is provided with a shutter member 78 to open andclose the writing access opening 75, reading/writing access opening 76and central opening 77 at the same time. The shutter member 78 isdimensioned differently from the shutter member of the disc cartridge 51but constructed similarly to the latter. So, the same or similarelements as or to those of the shutter member of the disc cartridge 51are indicated with the same reference numerals used for the latter andwill not be described in further detail.

[0122] The cartridge 74 of the disc cartridge 71 has formed on the frontside thereof an engagement recess 84 in which a shutter releasing memberprovided in a recording and/or reproducing apparatus is engaged when thedisc cartridge 71 is set in the recording and/or reproducing apparatus.The engagement recess 84 is open at one end thereof at which the disccartridge 71 is first introduced into the recording and/or reproducingapparatus.

[0123] When the disc cartridge 71 thus constructed is introduced into arecording and/or reproducing apparatus first at one lateral side thereofat which the engagement recess 84 is open, the shutter releasing member(not shown) of the recording and/or reproducing apparatus enters theengagement recess 84, and presses to displace the locking member (notshown), thereby disengaging the latter from the locking piece (notshown) of the shutter member 78. As the disc cartridge 71 is furtherinserted into the recording and/or reproducing apparatus after thelocking piece is disengaged from the locking member, the cartridge 74 ismoved while the shutter member 78 is restricted by the shutter releasingmember from moving, so that the writing access opening 75,reading/writing access opening 76 and central opening 77 are opened.Thus the disc cartridge 71 is set in place in the recording and/orreproducing apparatus.

[0124] As shown in FIG. 15, the cartridge 74 has formed on the lowersurface thereof first and second positioning holes 85 and 86 in whichpositioning pins provided in the recording and/or reproducing apparatusare engaged when the disc cartridge 71 is set in the recording and/orreproducing apparatus. The positioning holes 85 and 86 are formed inpositions at the front comers, respectively, of the cartridge 74 atwhich the holes 85 and 86 are not opposite to the optical disc 41encased in the cartridge 74.

[0125] The disc cartridge 71 is designed smaller than the disc cartridge51 since it is used to encase the optical disc 41 smaller than theoptical disc 1. However, since the optical discs 1 and 41 are identicalto each other in the method of attaching each of them to the rotationdriving mechanism of a recording and/or reproducing apparatus and themethod of writing and reading an information signal to and from theirsignal recording layers, it is desirable for both the disc cartridges 51and 71 to be usable commonly in the recording and/or reproducingapparatus which is compatible with at least the disc cartridge 51.

[0126] For selective use of the disc cartridges 51 and 71 of differentsizes in a common recording and/or reproducing apparatus, it isnecessary to provide the apparatus with a cartridge receiver or holderwhich can selectively position each of the disc cartridges 51 and 71 forattachment to the rotation driving mechanism.

[0127] A recording and/or reproducing apparatus for optical discs asrecording media comprises an optical pickup unit and magnetic head unitmoving over the upper and lower surfaces of the optical disc. Therefore,it is difficult to additionally provide a disc cartridge positioningmechanism within the reach of the optical pickup unit and magnetic headunit.

[0128] In such a situation, it should desirably be possible with only aminor modification of a recording and/or reproducing apparatus in whichthe disc cartridge 51 is usable, to set the disc cartridge 71 in placein the apparatus in a similar manner to that for setting the disccartridge 51.

[0129] To this end, the present invention provides also an adapter foruse with both the disc cartridges 51 and 71 commonly in a recordingand/or reproducing apparatus. FIGS. 16 to 18 show together the adapter.The adapter is generally indicated with a reference 91 and has the samesize as the disc cartridge 51.

[0130] As shown in FIG. 16, the adapter 91 is designed to have arectangular shape whose outside dimensions are same as those of thecartridge 54 of the disc cartridge 51. The adapter 91 has formed in thecenter thereof a cartridge holding concavity 92 to receive the disccartridge 71. The concavity 92 has a sufficient size to receive the disccartridge 71. The adapter 91 has formed, on the bottom of the concavity92, first and second positioning pins 93 and 94, which engage the firstand second positioning pin engagement holes 85 and 86, respectively,formed in the disc cartridge 71, as shown in FIGS. 16 and 17. Further,first and second projections 95 and 96 are formed on the bottom of theconcavity 92 along a wall of the concavity 92 opposite to the wall alongwhich the pins 93 and 94 are provided. The first and second projections95 and 96 are provided to support the lower surface of the disccartridge 71 and to level the latter in relation to the adapter 91.

[0131] Moreover, there is formed in the bottom of the concavity 92 anopening 97 through which the reading/writing access opening 76 andcentral opening 77, which is formed in the lower half of the disccartridge 71 received in the adapter 91, are exposed.

[0132] Furthermore, the adapter 91 has provided at the front sidethereof a shutter coupling member 98 which is connected to the shuttermember 78 of the disc cartridge 71 set in the adapter 91. The shuttercoupling member 98 is provided with an engagement projection 99 forengagement in the engagement hole 61 a formed in the juncture 61 of theshutter member 78. The shutter member 78 is installed movably along thefront side of the adapter 91 in the directions of arrows C and D in FIG.16, which is the same as the moving direction of the shutter member 78.The shutter coupling member 98 is also provided with an engagement piece100, which will be engaged with the shutter releasing member engagementrecess 84 formed in the disc cartridge 71 set on the cartridge holdingconcavity 92.

[0133] Further, the adapter 91 has formed along the front side thereof ashutter releasing member engagement recess 101 similar to the shutterreleasing member engagement recess 64 formed along the front side of thedisc cartridge 51.

[0134] Moreover, the adapter 91 has formed on the lower surface thereoffirst and second positioning pin engagement holes 102 and 103 inpositions, respectively, corresponding to the first and secondpositioning pin engagement holes 65 and 66 of the disc cartridge 51.

[0135] As shown in FIG. 18, the disc cartridge 71 is received in theadapter 91 thus constructed with the engagement projection 99 engaged inthe engagement hole 61 a of the shutter member 78, the engagement piece100 engaged in the shutter releasing member engagement recess 84 and thefirst and second positioning pins 93 and 94 engaged in the first andsecond engagement holes 85 and 86, respectively. The disc cartridge 71thus received in the adapter 91 will have the same shape as the disccartridge 51. Namely, it can be handled similarly to the disc cartridge51.

[0136] That is to say, the adapter 91 with the disc cartridge 71 set inthe cartridge holding concavity 92 is introduced, first at the lateralside thereof perpendicular to the front side on which the shuttercoupling member 98 is installed, into a recording and/or reproducingapparatus. As the adapter 91 is further inserted into the recordingand/or reproducing apparatus, the shutter releasing member provided onthe recording and/or reproducing apparatus enters the shutter releasingmember engagement recess 101 to move the shutter coupling member 98 inthe direction of arrow C in FIG. 18. As the shutter coupling member 98is moved in the direction of arrow C in FIG. 18, the engagement piece100 provided on the shutter coupling member 98 moves in the shutterreleasing member engagement recess 84 of the disc cartridge 71 in thedirection of arrow C to press and displace the locking member providedinside the disc cartridge 71. Thus the locking member is disengaged fromthe locking piece provided on the shutter member 78. As the adapter 91is inserted further into the reading and/or reproducing apparatus afterthe disengagement of the locking member from the locking piece, thecartridge 74 is moved along with the adapter 91 with the shutter member78 restricted by the engaging piece 100 from moving, so that the writingaccess opening 75, reading/writing access opening 76 and central opening77 are opened. Thereafter, the adapter 91 is set together with the disccartridge 71 held therein onto a cartridge mount in the recording and/orreproducing apparatus. The adapter 91 is correctly positioned on thecartridge mount since positioning pins provided in the recording and/orreproducing apparatus are engaged in the first and second positioningpin engagement holes 102 and 103, respectively.

[0137] It should be noted that to selectively set the disc cartridges 51and 71 of different sizes in the recording and/or reproducing apparatus,a cartridge mount in which each of the disc cartridges 51 and 71 can beheld may be provided in the apparatus. In this case, the cartridge mountis provided with positioning pins which are to be engaged in the firstand second positioning pin engagement holes 65 and 66 in the disccartridge 51, respectively, and those which are to be engaged in thefirst and second positioning pin engagement holes 85 and 86,respectively. To make it possible to selectively set the disc cartridges51 and 71 in the recording and/or reproducing apparatus, there areformed in the disc cartridge 51, larger than the disc cartridge 71,relief holes in which the positioning pins are to be received in thefirst and second positioning pin engagement holes 85 and 86,respectively, in the disc cartridge 71.

[0138] As having been described in the foregoing, since the disccartridges 51 and 71 encase the write/read optical discs 1 and 41,respectively, which are capable of recording an information signal, thecartridges 54 and 74 have formed in the upper half thereof the writingaccess openings 55 and 75 through which a magnetic head writesinformation to the respective exposed optical discs 1 and 41. However, adisc cartridge in which the read-only optical disc 111 is encased has nowriting access opening formed in the upper half thereof, while havingformed in the lower half thereof only a reading access opening, throughwhich the optical pickup unit is exposed and a central opening throughwhich the table-abutment thereof is exposed. Therefore, the disccartridge encasing the read-only optical disc may have only a shutterfor the reading access opening and central opening formed in the lowerhalf thereof.

[0139] The disc cartridge 51 encasing the optical disc 1 and the disccartridge 71 encasing the optical disc 41 are set in a recording and/orreproducing apparatus constructed as will be described below, to writeand read information signal to and from the optical discs 1 and 41.

[0140] As mentioned above, the disc cartridge 71 is set in the adapter91 and set as it is in the recording and/or reproducing apparatus.Similarly to the disc cartridges 51 and 71, the cartridge in which theread-only optical disc 111 is encased may be set in the recording and/orreproducing apparatus to read information signal recorded on the opticaldisc 111.

[0141] Referring now to FIG. 19, there is schematically displayed arecording and/or reproducing apparatus in which each of the disccartridges 51 and 71 and a disc cartridge encasing the read-only opticaldisc 111 is usable. The recording and/or reproducing apparatus isgenerally indicated with a reference 200. As shown, the disc cartridge51, the disc cartridge 71 set in the adapter 91 or the disc cartridgeencasing the read-only optical disc 111 is set in the recording and/orreproducing apparatus 200 to write information signals such as audiosignal, video signal, etc. and read an information signal recorded inthe optical disc. The recording and/or reproducing apparatus 200comprises a magnetic head unit to apply an external magnetic field tothe magneto-optical disc when writing information signal to the latter.

[0142] As shown in FIG. 19, the recording and/or reproducing apparatus200 comprises a housing, a flat, generally rectangular base 201 formedfrom a metallic material and disposed in the housing, and a cartridgeholder 202 to hold a disc cartridge. The cartridge holder 202 issupported pivotably on the base 201. The base 201 has incorporatedtherein a rotation driving mechanism 11 which drives to spin the opticaldisc 1 (41 or 111) and an optical pickup unit 203. There is provided onthe base 201 a cartridge mount 205 on which a disc cartridge (not shown)held in the cartridge holder 202 is set. The cartridge holder 202 hasdisposed thereon a magnetic head unit 206 which is moved synchronouslywith the optical pickup unit 204 radially of the optical disc 1.

[0143] The rotation driving mechanism 11 is constructed as havingpreviously been described with reference to FIG. 4.

[0144] As shown in FIG. 19, the optical pickup unit 204 comprises mainlya semiconductor laser to emit a light beam, an objective lens 131 tofocus the light beam emitted from the semiconductor laser onto theoptical disc 1, a photodetector to detect a return light from theoptical disc 1 in order to detect an information signal and controlsignals, etc. The optical pickup unit 204 is supported at both sidesthereof by a pair of guide shafts 207 installed in parallel to eachother on the bottom of the base 201 to be movable radially of theoptical disc 1 set on the disc table 14. The objective lens 131 facesthe upper surface of the base 201 and the optical disc 1 set on the disctable 14 through an opening 208 formed in the base 201. The opticalpickup unit 204 is guided on the guide shafts 207 by a feed screw (notshown) driven by a drive motor (not shown) fixed on the bottom of thebase 201, and thus moved radially of the optical disc 1 on the disctable 14. That is, to write or read an information signal, the opticalpickup unit 204 is moved radially from the lead-in area towards thelead-out area of the optical disc 1.

[0145] As shown also in FIG. 19, the magnetic head unit 206 is linkedwith the optical pickup unit 204, which writes an information signal tothe optical disc 1. Namely, the magnetic head unit 206 is also movedradially of the optical disc I on the disc table 14 as the opticalpickup unit 204 is so moved. The magnetic head unit 206 comprises amagnetic head 210 to apply to the optical disc 1 with an externalmagnetic field modulated according to an information signal to bewritten to the optical disc, and a magnetic head supporting plate 211provided with a leaf spring supporting at the free end thereof themagnetic head 210 and which can elastically be displaced. The magnetichead unit 206 is linked with the optical pickup unit 204 by a linkagemember 213 in such a manner that it is positioned in a position where itwill be opposite to the optical pickup unit 204 with the optical disc 1on the disc table 14 located between them.

[0146] As shown in FIG. 19, the linkage member 213 consists of acoupling piece 215 and a fixture 216 formed by bending one end of thecoupling piece 215 perpendicularly to the latter itself and by which thelinkage member 213 is fixed to the optical pickup unit 204. The linkagemember 213 is formed to have a generally L shape. The linkage member 213is installed to the optical pickup unit 204 with the fixture 216 fixedto the optical block with fixing screws or the like so that the free endof the coupling piece 215 projects above the base 201. The base end ofthe magnetic head supporting plate 211 is pivotably supported on theother end of the coupling piece 215 by means of a pivot (not shown).Thus the magnetic head supporting plate 211 is pivotable about the pivotin directions of the magnetic head 210 supported at the free end of theplate 211 being turned towards and away from the optical disc 1 on thedisc table 14.

[0147] As the optical pickup unit 204 is driven by the feeding motor tomove radially of the optical disc 1, the magnetic head unit 206 thuscoupled to the optical pickup unit 204 by means of the linkage member213 is also moved along with the optical pickup unit 204 radially of theoptical disc 1.

[0148] As shown in FIG. 19, the cartridge holder 202 in which the disccartridge is inserted and held is pivotally installed to the base 201 ina position between the base 201 and magnetic head unit 206. Thecartridge holder 202 has provided at opposite lateral sides thereofcartridge guides 221 and 222 to guide and hold a disc cartridge, andalso has a cartridge inlet 223 formed at the front side thereof. A disccartridge inserted from the cartridge inlet 223 into the cartridgeholder 202, is held at opposite lateral sides thereof by the cartridgeguides 221 and 222, respectively.

[0149] The recording and/or reproducing apparatus 200 with the rotationdriving mechanism 11 constructed as mentioned above works as will bedescribed below when the disc cartridge 51 encasing the optical disc 1,for example, is set therein:

[0150] First, to write an information signal to the optical disc 1 or toread an information signal recorded on the optical disc 1, the cartridgeholder 202 is pivoted to its upper position where it is apart from thecartridge mount 205. At this time, the cartridge inlet 223 of thecartridge holder 202 will be directed to above the apparatus. Then, thecartridge holder 202 in which the disc cartridge is held is pivotedtowards the cartridge mount 205 and thus the disc cartridge 51 ispositioned in place on the cartridge mount 205.

[0151] At this time, the optical disc 1 in the disc cartridge 51 is seton the disc table 14 of the rotation driving mechanism 11 and clampedthere for rotation with the disc table 14. When it is detected that thedisc cartridge set on the cartridge mount 205 encases the optical disc 1and the write mode of operation is selected by using a record button,the spindle motor 12 is driven to spin forward and the optical disc 1 isspun along with the disc table 14. The optical pickup unit 204 is putinto operation. A light beam emitted from a light source scans over thesignal recording layer 21 of the optical disc from the lead-in to leadout while the magnetic head 210 applies the optical disc 1 with anexternal magnetic field modulated according to an information signal tobe written. Thus, a desired information signal is written to the opticaldisc 1.

[0152] When the read mode of operation is selected by using a playbutton, a light beam emitted from the semiconductor laser of the opticalpickup unit 204 is irradiated to the signal recording layer 21 of theoptical disc 1, a return light from the signal recording layer 21 of theoptical disc 1 is detected by the photodetector of the optical pickupunit 204. Thus, an information signal is read from the optical disc 1.It should be noted that for information signal reading, no externalmagnetic field may be applied to the optical disc 1, namely, themagnetic head unit 206 is not required.

[0153] In the above-mentioned recording and/or reproducing apparatus200, the optical pickup unit 204 emits a light beam which enables a highdensity recording as having previously been described. A semiconductorlaser emitting a light beam of 380 to 420 nm in wavelength λ or a lightbeam of 630 to 670 nm in wavelength λ is used as the laser source. Theobjective lens 131 has a numerical aperture NA of 0.5 to 0.7 to permit ahigh resolution. The magnetic head 210 is a high frequency type one toenable a high density recording.

[0154] In addition to the first disc cartridge 51 encasing the firstoptical disc 1, the second disc cartridge 71 encasing the second opticaldisc 41 or the disc cartridge in which the read-only optical disc 111 isencased can be used in the recording and/or reproducing apparatus towrite or read an information signal in the same manner as mentionedabove.

[0155] As having been described in the foregoing, since the opticalrecording medium and the disc cartridge encasing the optical recordingmedium according to the present invention are very compact and able toan record information signal with a high density, a recording and/orreproducing apparatus can be provided which is correspondingly compactand can write or read an information signal such as audio and videoinformation for a sufficiently long time.

[0156] Although modifications and changes may be suggested by those ofordinary skill in the art, it is the intention of the inventors toembody within the patent warranted hereon all changes and modificationsas reasonably and properly come within the scope of their contributionto the art.

We claim as our invention:
 1. An optical read/write recording mediumcomprising: a disc having a diameter of 65 mm or less and a thicknessbetween and including 0.4 mm and 0.7 mm, the disc further having acenter; and a recording area for recording an information signal on thedisc, the recording area extending outwardly 12.5 mm or less from thecenter of the disc, a ratio in diametrical length between the recordingarea and a non-recording area on the disc being 3.4 or more, saidrecording area having an information signal storage capacity of at least2 Gigabytes.
 2. The optical read/write recording medium according toclaim 1, wherein said recording area further comprises tracks having apitch between and including 0.3 μm and 0.55 μm for recording aninformation signal represented as pits in said tracks, said pits havinga length between and including 0.1 μm/bit and 0.22 μm/bit.
 3. Theoptical read/write recording medium according to claim 1, furthercomprising: a center hole formed at the center of the disc; an annulartable-abutment formed around the center hole and being convex to oneside of the disc; and a magnetic-clamping hub fixed to the annulartable-abutment.
 4. The optical read/write recording medium according toclaim 3, wherein the annular table-abutment is convex by 0.4 mm to 0.7mm and has an outside diameter of about 14 mm and inside diameter ofabout 8 mm.
 5. An optical read/write recording medium comprising: a dischaving a diameter between 38 mm and 52 mm and a thickness between andincluding 0.4 mm to 0.7 mm, said disc having a center; and a recordingarea on the disc extending outwardly 12.5 mm or less from the center ofthe disc, said recording area having an information signal storagecapacity of at least 1 Gigabyte.
 6. The optical read/write recordingmedium according to claim 5, wherein the recording area furthercomprises tracks having a pitch between and including 0.3 μm and 0.55 μmfor recording an information signal represented as pits in the tracks,the pits having a length between and including 0.1 μm/bit and 0.22μm/bit.
 7. The optical read/write recording medium according to claim 5,further comprising: a center hole formed at the center of the disc; anannular table-abutment formed around the center hole and being convex toone side of the disc; and a magnetic-clamping hub fixed to the annulartable-abutment.
 8. The optical read/write recording medium according toclaim 7, wherein the table-abutment is formed to a same size as that ofa second table-abutment formed for a second optical recording mediumcomprising: a disc having a diameter of 65 mm or less and a thicknessbetween and including 0.4 mm and 0.7 mm, the disc having a center; and arecording area for recording an information signal on the disc, therecording area extending outwardly 12.5 mm or less from the center ofthe disc, a ratio in diametrical length between the recording area and anon-recording area on the disc being 3.4 or more, said recording areahaving an information signal storage capacity of at least 2 Gigabytes.9. A read-only optical recording medium simulated to a read/writerecording medium, the read/write recording medium having a disc with adiameter of 65 mm or less and a thickness between and including 0.4 mmand 0.7 mm, a recording area on the disc extending outwardly 12.5 mm orless from a center of the disc, the recording area having a storagecapacity of at least 2 Gigabytes and a ratio in diametrical lengthbetween the recording area and a non-recording area of the disc being3.4 or more, the read-only optical recording medium comprising: asubstrate formed to a disc shape having a thickness between andincluding 0.4 mm and 0.6 mm by injecting a resin into a mold havingformed thereon a pit pattern on a recording track in which a smallestpit length and a recording track pitch are about 1.4 times larger than apit length and a track pitch on the read/write recording medium; a fistinformation recording layer formed on the pit pattern on the substrate;a light-transparent sheet between and including 0.05 mm and 0.1 mm inthickness and having on one side thereof a pit pattern formed bythermally stamping a pit-pattern stamper; and a second informationrecording layer formed on the pit-pattern side of the light-transparentsheet, wherein the pit pattern on the substrate and pit pattern on thelight-transparent sheet join each other with a transparent resin betweenthem, the transparent resin between the substrate and light-transparentsheet having a length between and including 20 μm and 35 μm.
 10. Theread-only optical recording medium according to claim 9, wherein thefirst information recording layer is translucent for optically reading arecorded information signal from the substrate side.
 11. The read-onlyoptical recording medium according to claim 9, further comprising: acenter hole formed at the center of the substrate; an annulartable-abutment formed around the center hole and being convex to oneside of the substrate; and a magnetic-clamping hub fixed to the annulartable-abutment.
 12. The read-only optical recording medium according toclaim 11, wherein the annular table-abutment in convex by 0.4 mm to 0.7mm and has an outside diameter of about 14 mm and inside diameter ofabout 8 mm.
 13. A read-only optical recording medium simulated to aread-write recording medium, the read-write recording medium including adisc having a diameter between 38 mm and 52 mm and a thickness betweenand including 0.4 mm and 0.7 mm, a recording area on the disc extendingoutwardly 12.5 mm or less from a center of the disc, the recording areahaving an information signal storage capacity of at least 1 GB, theread-only optical recording medium comprising: a substrate formed to adisc shape having a thickness between and including 0.4 mm and 0.6 mm byinjecting a resin in a mold having formed thereon a pit pattern on arecording track in which a smallest pit length and a recording trackpitch are about 1.4 times larger than a pit length and a track length onthe read-write recording medium; a fist information recording layerformed on the pit pattern on the substrate; a light-transparent sheetbetween and including 0.05 mm and 0.1 mm in thickness and having on oneside thereof a pit pattern formed by thermally stamping a pit-patternstamper; and a second information recording layer formed on thepit-pattern side of the light-transparent sheet, wherein the pit patternon the substrate and pit pattern on the light-transparent sheet joineach other with a transparent resin between them, the transparent resinbetween the substrate and light-transparent sheet having a lengthbetween and including 20 μm and 35 μm.
 14. The read-only opticalrecording medium according to claim 13, wherein the first informationrecording layer is translucent for optically reading a recordedinformation signal from the substrate side.
 15. The read-only opticalrecording medium according to claim 13, a center hole formed at thecenter of the substrate; an annular table-abutment formed around thecenter hole and being convex to one side of the substrate; and amagnetic-clamping hub fixed to the annular table-abutment.
 16. Theread-only optical recording medium according to claim 13, wherein theannular table-abutment is convex by 0.4 mm to 0.7 mm and has an outsidediameter of about 14 mm and inside diameter of about 8 mm.
 17. A disccartridge in which a read-write recording medium is encased, theread-write recording medium including a disc having a diameter of 65 mmor less and a thickness between and including 0.4 mm and 0.7 mm, arecording area on the disc extending outwardly 12.5 mm or less from acenter of the disc, the recording area having a storage capacity of atleast 2 Gigabytes, a ratio in diametrical length between the recordingarea and a non-recording area of the disc being 3.4 or more, the disccartridge comprising: a cartridge having a rectangular shape andincluding a first side having length between and including 52 mm and 70mm, and a second side, perpendicular to the first side, having a lengthbetween and including 52 mm and 75 mm, the cartridge having a thicknessbetween and including 3 mm and 6 mm; and at least one opening formed ina side of the cartridge parallel to the read/write recording medium, theopening being a writing and reading access opening through which aradially extending portion of the recording medium is exposed to a lightbeam from an optical pickup and through which the magnetic-clamping hubis exposed, the opening further being formed deviated to one lateralside from a middle point of a front side of the cartridge.
 18. The disccartridge according to claim 17, further comprising: a shutter memberslidably mounted along one lateral side of the cartridge for opening andclosing at least the writing access opening and reading access openingformed in the cartridge.
 19. A disc cartridge having a read-writerecording medium is encased therein, the read-write recording mediumincluding a disc having a diameter between 38 mm and 52 mm and athickness between and including 0.4 mm and 0.7 mm, a recording area onthe disc extending outwardly 12.5 mm or less from a center of the disc,the recording area having a storage capacity of at least 1 Gigabyte, thedisc cartridge comprising: a cartridge having a rectangular shape andincluding a first side having a length between and including 41 mm and57 mm and a second side, perpendicular to the first side, having alength between and including 41 mm and 62 mm, the cartridge having athickness between and including 3 mm and 6 mm; and at least an openingformed in a side of the cartridge parallel to the read-write recordingmedium, the opening being a writing and/or reading access openingthrough which a radially extending portion of the recording medium isexposed to a light beam from an optical pickup and through which themagnetic-clamping hub is exposed, the opening further being formeddeviated to one lateral side from a middle point of a front side of thecartridge.
 20. The disc cartridge according to claim 19, furthercomprising: a shutter member slidably mounted along one lateral side ofthe cartridge for opening and closing at least the writing accessopening and reading access opening formed in the cartridge.
 21. A disccartridge having a read-only optical recording medium encased therein,the read-only optical recording medium including a disc having adiameter of 65 mm or less and a thickness between and including 0.4 mmand 0.7 mm, a recording area on the disc extending outwardly 12.5 mm orless from a center of the disc, the recording area having a storagecapacity of at least 2 Gigabytes, a ratio in diametrical length betweenthe recording area and a non-recording area of the disc being 3.4 ormore, the disc cartridge comprising: a cartridge of a rectangular shapeincluding a first side having a length between and including 52 mm and70 mm and a second side, perpendicular to the first side, having alength between and including 52 mm and 75 mm, the cartridge having athickness between and including 3 mm and 6 mm; and at least one openingformed in a side of the cartridge parallel to the read-only recordingmedium, the opening being a writing and reading access opening throughwhich a radially extending portion of the recording medium is exposed toa light beam from an optical pickup through which the magnetic-clampinghub is exposed, the opening further being formed deviated to one lateralside from a middle point of a front side the cartridge.
 22. The disccartridge according to claim 21, further comprising: a shutter memberslidably mounted along one lateral side of the cartridge for opening andclosing at least the writing access opening and reading access openingformed in the cartridge.
 23. A disc cartridge having a read-only opticalrecording medium encased therein, the read-only optical recording mediumincluding a disc having a diameter of between 38 mm and 52 mm and athickness between and including 0.4 mm and 0.7 mm, a recording area onthe disc extending outwardly 12.5 mm or less from a center of theread-only optical recording medium, the recording area having a storagecapacity of at least 1 Gigabyte of an information signal as a pitpattern, the disc cartridge comprising: a cartridge of a rectangularshape including a first side having a length between and including 41 mmand 52 mm and a second side, perpendicular to the first side, having asecond length between and including 41 mm and 62 mm, the cartridgehaving a thickness between and including 3 mm and 6 mm; and at least anopening formed in a side parallel to the read-write recording medium,the opening being a writing and/or reading access opening through whicha radially extending portion of the optical disc is exposed to a lightbeam from an optical pickup and through which the magnetic-clamping hubis exposed, the opening being formed deviated to one lateral side from amiddle point of a front side of the cartridge.
 24. The disc cartridgeaccording to claim 23, further comprising: a shutter member slidablymounted along one lateral side of the cartridge for opening and closingat least the writing access opening and reading access opening formed inthe cartridge.